1. Field of the Invention
The present invention relates to a method for manufacturing an external force detection device used for an acceleration sensor and the external force detection device.
2. Description of the Related Art
As shown in a sectional view of FIG. 5D, a device including a weight portion 7, a supporting portion 5 arranged so as to be spaced apart from the weight portion 7, and a beam portion 6 that connects the supporting portion 5 to the weight portion 7 at one or more connecting points (one connecting point in FIG. 5D) is known as one example of an external force detection device used for an acceleration sensor or other suitable device. The beam portion 6 is configured so as to be thinner than the weight portion 7. As shown in FIGS. 5D and 5E, a space in which the weight portion 7 can move is provided around the weight portion 7. In this type of an external force detection device, the weight portion 7 is displaced in accordance with an external force to cause the beam portion 6 to deform. Such an external force detection device is used for various devices, such as an acceleration sensor as disclosed in Japanese Patent No. 3956999 and Japanese Unexamined Patent Application Publication Nos. 2004-125616 and 2004-109114, for example.
The weight portion 7, the supporting portion 5, and the beam portion 6 are provided in, for example, a silicon-on-insulator (SOI) substrate. As shown in FIG. 5A, an SOI substrate 1 is a multi-layered substrate in which a lower layer 4 (supporting layer) made of silicon (Si), an intermediate layer 3 (BOX layer) made of silicon oxide (SiO2), and an upper layer 2 (SOI layer) made of Si are stacked in sequence. In other words, the upper layer 2 and the lower layer 4 sandwich the intermediate layer 3.
The upper layer 2 and the lower layer 4 are made of Si and can be etched with a first etching gas that is capable of removing Si. On the other hand, the intermediate layer 3 is made of SiO2 and can be etched with a second etching gas different from the first etching gas, but cannot be etched with the first etching gas.
For example, the formation of the weight portion 7, the supporting portion 5, and the beam portion 6 using the SOI substrate 1 is described with reference to FIGS. 5B to 5D. As shown in FIG. 5B, a groove 9 is formed by etching, with the first etching gas, the upper layer 2 in a gap portion 8 between the weight portion 7 and the supporting portion 5 that define the external force detection device (a step of etching an upper layer). As shown in FIG. 5E, the groove 9 is formed in a straight line.
As shown in FIG. 5C, the lower layer 4 in the gap portion 8 and the lower layer 4 below the beam portion 6 are etched with the first etching gas (a step of etching a lower layer). The lower layer 4 in the gap portion 8 is etched such that the etched portion is wider than the groove 9. As a result, the weight portion 7 is connected to the supporting portion 5 through the beam portion 6 (e.g., cantilevered in this drawing), and only the intermediate layer 3 remains unetched in the gap portion 8 between the supporting portion 5 and the weight portion 7.
As shown in FIG. 5D, the intermediate layer 3 in the gap portion 8 is then etched with the second etching gas (a step of etching an intermediate layer), whereby the weight portion 7 is separated from the supporting portion 5 through the gap portion 8. Furthermore, the intermediate layer 3 below the beam portion 6 is optionally etched with the second etching gas. Consequently, the weight portion 7, the beam portion 6, and the supporting portion 5 of the example of an external force detection device described above can be formed.
In the step of etching a lower layer shown in FIG. 5C, the lower layer 4 is gradually etched from the lower side thereof (that is, from the lower side of the SOI substrate 1). However, the unstable etching rate causes a state in which the lower layer 4 on the intermediate layer 3 that remains unetched has a non-uniform thickness. If a groove 9 with a simple straight line shown in FIG. 5E is formed above the intermediate layer 3, stress is non-uniformly applied to the intermediate layer 3. As a result, the stress is concentrated in a region where the groove 9 is formed, represented by arrows F in FIG. 5E.
The intermediate layer 3 may be locally ruptured at that region before the step of etching an intermediate layer. The shock caused by the rupture may break off the beam portion 6, which may also cause the weight portion 7 to jut out from the substrate. Thus, the manufacturing yield of an external force detection device may be reduced or an external force detection device that does not conform to standards may be produced.